This invention relates to carbon-nitrogen nanofibers and methods for their preparation.
Carbon nanofibers are known in the art, their preparation being described in a large number of patents and other literature. They are typically cylindrical tubes with graphitic walls of a diameter between about 3.5 and about 500 nm, having a hollow core which may be filled with amorphous carbon. They are commercially available from Hyperion Catalysts International, Inc., and are useful as fillers for resinous materials, and often impart conductivity to said materials.
Carbon nanofibers may be prepared by the catalytic reaction of a hydrocarbon gas with hydrogen in the presence of a transition metal catalyst. Typical reaction temperatures are in the range of about 500-1,500xc2x0 C. The size of the fibers is largely dependent on the particle size of the catalyst employed. Typical methods of preparing such nanofibers are disclosed, for example, in U.S. Pat. Nos. 4,663,230, 5,165,909 and 5,500,200.
The microstructures of the nanofibers most often obtained according to the prior art are of two types. In the first microstructure, hereinafter sometimes designated xe2x80x9cfishbonexe2x80x9d, the layers of graphite on the surface of a nanofiber emanate from a core region and are parallel to each other but not parallel to the length axis of the nanofiber. In the second microstructure, hereinafter sometimes designated xe2x80x9cparallelxe2x80x9d, the graphite layers are parallel both to each other and to the length axis of the nanofiber. Typical d-spacings (i.e., distances between the centers of carbon layers) are on the order of 3.43 xc3x85 for the fishbone microstructure and 3.47-3.50 xc3x85 for the parallel microstructure, as determined by x-ray scattering.
Similar nanofibers, also designated xe2x80x9cnanotubesxe2x80x9d, comprising carbon and nitrogen have been reported in Sen et al. Chemical Physics letters, 287, 671-676 (1998). They were prepared by bubbling argon gas through a carbon-nitrogen compound such as pyridine and conducting the resulting vapors over a cobalt powder catalyst at 750xc2x0 C.
These nanofibers exhibit variability in microstructure, having many defects that suggest formation by a mechanism similar to the xe2x80x9cslip-stickxe2x80x9d phenomenon encountered in adhesion phenomena. Such variability can affect the performance of the nanofibers as fillers and conductive materials.
It is of interest, therefore, to produce carbon-nitrogen nanofibers having a more regular microstructure than that demonstrated in the prior art materials.
The present invention includes carbon-nitrogen nanofibers having a fishbone microstructure. Also included is a method for preparation of such nanofibers.
One aspect of the invention is compositions comprising carbon-nitrogen nanofibers in which the structural layers emanate from a core catalyst particle and are parallel to each other but not parallel to the length axis of the nanofibers.
Another aspect is a method for preparing a carbon-nitrogen nanofiber composition which comprises contacting, at a temperature in the range of about 500-1000xc2x0 C., a gaseous mixture comprising hydrogen and a carbon-nitrogen compound with a catalyst comprising cobalt and iron.
Still another aspect is resinous compositions comprising a thermoplastic or thermosetting resin as the continuous phase and a carbon-nitrogen nanofiber composition as described hereinabove.